238 ‘THE ORIGIN OF VERTEBRATES 
I conclude, then, that the cheliceree must truly be included in 
the prosomatic group, but that they stand in a somewhat different 
category to the rest of the prosomatic appendages, inasmuch as they 
take up a very median anterior and somewhat dorsal position, and 
their ganglia of origin are also exceptional in position. 
Next for considération come the chilaria (7 in Fig. 103), which 
Lankester did not consider to belong to appendages at all, but to 
be a peculiar pair of sternites. Yet their very appearance, with 
their spinous hairs corresponding to those of the other gnathites and 
their separate nerve-supply, all point distinctly to their being a 
modified pair of appendages, and, indeed, the matter has been placed 
beyond doubt by the observations of Kishinouye, who has found 
embryologically that they arise in the same way as the rest of the 
prosomatic appendages, and belong to a distinct prosomatic segment, 
viz. the seventh segment. In accordance with this, Brauer has found 
that in the scorpion there is in the embryo a segment, whose ap- 
pendages degenerate, which is situated between the segment bearing 
the last pair of thoracic appendages and the genital operculum—a 
segment, therefore, comparable in position to the chilarial segment of 
Limulus. 
Coming now to the five locomotor appendages, we find that they 
resemble each other to a considerable extent in most cases, with, 
however, certain striking differences. Thus in Limulus they are 
chelate, with their basal joints formed as gnathites, except in the 
case of the fifth appendage, in which the extremity is modified for 
the purpose of digging in the sand. In Pterygotus, Slimonia, Euryp- 
terus, the first four of these appendages are very similar, and are 
called by Huxley and Woodward endognaths; in all cases they 
possess a basal part or sterno-coxal process, which acts as a gnathite 
or foot-jaw, and a non-chelate tactile part, which possesses no pre- 
hensile power, and in most cases could have had no appreciable 
share in locomotion, called by Huxley and Woodward the palpus. 
These small palps were probably retractile, and capable of being 
withdrawn entirely under the hood. The fifth appendage is usually 
different, being a large swimming organ in Pterygotus, Eurypterus, 
and Slimonia (Figs. 8 and 104), and is known as the ectognath. 
Finally, in Drepanopterus Bembycoides, as stated by Laurie, all 
five locomotor appendages are built up after the same fashion, the. 
last one not being formed as a paddle-shaped organ or elongated as 
